I didn't see a discussion thread for it here so far, but I'd love to hear some EU thoughts on it as well!

Personally, I see it as a double edged sword. Depending on the effects an epic pet would have (higher stats, most likely?) it would change everything we know around pet battles, and that's both a scary and exciting outlook. In PvP I guess it would skew over PvP for a long time towards people who collected more epic pets already, until there is a competitive balance reached by the majority of players again. But once that's reached it could work very well.

On the PvE side things are different. Most of the old content is already trivialized by the abundance of strong pet combinations and guides. Making the fights a bit more easy would not change anything in the need to level up pets (for those who don't have 700+ lvl 25 ones), which is among the main time-sinks in pet battling.It would mean however that most people would like to upgrade their whole collection and getting 700+ of those stones, or even worse going back to re-capturing wild pets in epic quality and abandoning their blue ones, sounds like a horribly tedious (and boring) task. I would definitely not look forward to this, if that's all the epic quality brings.

Speaking of benefits then, what could this additional quality level bring? Additional skills for more variety? Maybe not higher stats but instead a buff that's different for each pet family, or a racial skill that can be activated once per fight?

First and foremost, thank you very much for testing pet battles in the Legion Alpha. We’ve heard your concerns and have decided not to move forward with epic pet battle stones in Legion.

To explain a bit about how we arrived at this point--

When designing rewards for the new battle pet features in Legion and the revamped pet store in Dalaran, we considered epic stones a potential direction to go. Some players are nearing the cap of max-level, all-rare pet teams, and we wanted to see if this space would satisfy their needs for a long-term future level of investment.

You have correctly identified a few of the issues that come up with this feature:

New players now have a harder time catching up (though we could mitigate this by speeding up the early leveling process).

Players with 600+ rare pets that felt as if they were nearing completion of their pet journal are back to having 600+ pets to level.

Power Creep – older content is now easier, future content is harder for new players entering pet battles.

One of the things we like about pet battles is that much of the early content stays relevant through all of the expansions. Similarly, the collection aspect of battle pets works well when it’s wider (more pets to collect, more variety in abilities) rather than taller (more levels). Both of these are very good reasons to avoid adding epic pet stones.

This direction also pushes us to create content that tests max-level-pet-battlers in a way that is not purely numbers, but use of abilities, use of specific pets, and memorization of enemy strategies. The Celestial Tournament was a good example of pet battle content that rewarded a varied and filled pet stable. We have some exciting things in mind for future battles versus tamers!

It’s worth noting that this feature was not intended to replace any gameplay content. We’re working on all sorts of pet battle updates that we’d like to make in future patches.

Thanks again for all of your feedback. We really appreciate your great ideas and discussion!